The Upper Arkansas River Valley has been through many geological changes. The Ancestral Rockies rose 325 million years ago (mya) and then eroded completely away, with seas moving inland and then receding several times. Around 65 mya the Laramide Orogeny pushed the land up again until it formed a new hump running from north to south, known as the Sawatch uplift. After 35 million years, stresses in the continental plate pulled at this hump from both sides until the center broke loose and slid downward. The fallen part created a trough that spanned from Leadville to New Mexico known as the Rio Grande Rift.
During this period 30 mya, volcanoes spewed lava and ash that formed mountains and hills along the eastern side of the rift where the Mosquito Range lies. Runoff from both sides formed the Arkansas River, which flowed south through the rift and joined the Rio Grande River. Volcanic activity continued and sealed-off the river near Poncha Pass, causing it to flow east instead of south, out of the mountains and through the canyons, across the Great Plains and eventually to the Mississippi River.
During three separate glacial episodes over the last one million years, and as recently as 10,000 years ago, glaciers scoured the Sawatch Range and filled the valley with deep layers of rocks and dirt. The masses of sediment drastically affected the course of the Arkansas River. The geologic action both blessed and cursed the upper Arkansas River valley with a wealth in gold, silver and semi-precious gems. Although the mining boom is over, many people still search for treasure in the Arkansas River and surrounding hills.
Today the mountains rise and erode at about the same speed, maintaining a near constant elevation. Magma below the surface of the mountains continues to fuel the local hot springs, and the Arkansas River continues to carve its channel.